For some time now (before 5.9.4 at least, update did not change anything) there are some glitches that I (and turns out not only me https://www.reddit.com/r/kde/comments/5zij0p/a_visual_glitch_in_the_task_switcher/) have found inf Task Switcher, Desktop Grid effect and when a window is being dragged for tiling into a specific area.

This happens when OpenGL 2.0/3.1 was selected. When I tried XRender (as the reddit entry suggests) and everything works as it used to. That's why I'm submitting the bug against KWin.

Screenshots: http://imgur.com/a/udzyD

For Task Switcher:
Task switching looks like on the screenshots 1 and 3. Expected behaviour is to have background and selection on the switcher visible. 

For Desktop Grid:
The buttons (or at least their space) for adjusting the number of virtual desktops is covered with a corner of a desktop. It can be seen on screenshot number 2. 'Browse' text is from full screen Spotify window. Sometimes there is a top panel with a part of the screen visible (so upper-left corner).

For window tiling:
When a window is dragged to an edge of the screen (no matter which, top/right/left) the area that is going to be covered with resized window should be painted (i.e. take the left half of the screen, upper-left quarter etc.). This does not happen and the only effect is shown on the last screenshot from the gallery.